This invention relates to a pneumatic radial tire.
In general, with a pneumatic radial tire, belt end separations (separations of plies at belt ends) often occur when large shearing forces act between plies at belt ends due to enlarging diameter when filling with inner pressure and deformation in contact road and rolling under loaded condition.
In order to prevent such a separation, it has been proposed to arrange a cap having a strong hoop effect radially outwardly of a belt layer to cover the entire width of the belt layer. Such a cap has cords embedded therein and extending substantially in parallel with an equatorial plane of the tire.
With such a pneumatic tire, however, as the cap covers the overall width of the belt layer, the belt layer is difficult to deform not only at a center portion of the width but also at both ends thereof in width directions. In this case, a crown portion of the pneumatic tire is generally convex as a whole so that a diameter at the center portion is larger than that at shoulder portions. Therefore, when the pneumatic radial tire is deformed in rolling and contacting a road under a loaded condition, the belt layer resists deformation so that it becomes difficult to absorb the difference in diameter between the center portion and the shoulder portions of the tread, with the result that contacting pressure at the shoulder portions with the road is lowered. As a result, the shoulder portions are likely to be dragged so that the shoulder portions are slid in the contacting area on the road. Accordingly, irregular wear such as shoulder wear would occur in the shoulder portions.
Inventors of this application have been investigated tires of this kind and proposed to narrow the width of the cap having cords embedded therein substantially in parallel with an equatorial plane to make easy the deformation of the both ends of the belt layer in width directions, thereby facilitating absorbing the difference in diameter between shoulder portions and the center portion when deforming in contacting the road. With this arrangement, however, although the shoulder wear above described can be prevented, the hoop effect at both ends of the belt layer is eliminated so that the shearing force between plies at ends of the belt plies is increased to a certain degree resulting in failure or trouble.
Moreover, although the irregular wear such as shoulder wear can be prevented, tread portions (ribs) in regions overlapping the reinforcing ply are sometimes conspicuously rapidly worn. Such a wear is referred to as "rib punch". This phenomenon results from the fact that if one circumferential length of the reinforcing ply is slightly long to have slacks, the reinforcing plies and the ribs in the region overlapping the reinforcing plies are waved, with the result that recesses of the ribs corresponding to bottoms of the waves are dragged, when contacting the road, to slip on the road resulting in rapid wear.